The present invention relates to ISDN (Integrated Services Digital Network) terminal equipment connected to an ISDN basic rate interface and, more particularly, ISDN terminal equipment capable of enhancing efficient communication.
ISDN terminal equipment for use with an ISDN is extensively used today and connected to an ISDN basic rate interface. The basic rate interface is assigned to independent subscriber lines which are accommodated in an existing telephone network and implemented by metallic subscriber lines. The basic rate interface is provided with two B channels for user data transfer and having a transmission rate of 64 kilobits per second (kb/s), and one D channel for signaling and data transfer and having a transmission speed of 16 kb/s. Regarding private wiring, the basic rate interface has a bus configuration selectively implementing 1:1 connection or 1:n connection, allowing a plurality of ISDN terminal equipment to share a single bus.
Assume that when one of a plurality of ISDN terminal equipment connected to a single bus originates a call, another ISDN terminal has already occupied the two B channels. Then, the basic rate interface returns a message representative of a no idle channel condition, i.e., a channel release message REL or a release complete message REL COMP to the calling terminal. In this condition, it has been customary for the terminal equipment to originate a call again on the elapse of a predetermined period of time by use of, e.g., a timer. However, determining the waiting time uniformly by a timer, i.e., neglecting the actual communication obstructs efficient communication.
Moreover, for the ISDN terminal equipment using the two B channels, once the B channels are occupied, they are continuously occupied until the end of communication, despite that a single B channel suffices the communication. This prevents the other terminal equipment connected to the same bus in a multipoint configuration from originating a call, thereby obstructing smooth communication. In addition, when the ISDN terminal equipment occupying the B channels receives a setup request SETUP from another terminal equipment sharing the same bus, it cannot respond to the request and causes the calling terminal to wait until the communication ends.